Electrochemical cells of the type formed in layers, including some layers laid down by printing, are adapted for such purposes as single-use power sources and self-powered displays. Electrode layers are ionically separated prior to use for extending shelf life and for providing an alternative mechanism for activating the cells.
Self-discharge is a problem of electrochemical cells, particularly for less expensive cells made by printing at least some of the active layers. The unwanted electrochemical activity occurs through an electrolyte layer connecting two electrode layers (i.e., an anode layer and a cathode layer). The electrolyte layer provides a pathway for ionic conduction between the electrodes. However, some electrical conductivity can also occur within the electrolyte layer, which short circuits the cells and supports the unwanted discharge.
Chemical and physical interactions of the electrolyte layer with its surroundings can also degrade electrochemical cell performance. For example, materials within the electrolyte layer can interact with one or more of the electrode layers to chemically alter the electrode layers. Evaporation of liquids from the electrolyte layer can significantly degrade the ionic conductivity of the electrolyte layer.
Coin cells are popular as low-power sources for voice-chips, such as those used in greeting cards, as well as for electrochromic or liquid crystal displays. However, such coin cells are quite bulky for use with printed products and require physical connections that add to their bulk and diminish their reliability. Even though the coin 30 cells are sealed to protect their active layers, self-discharge is still a problem, especially when the cells are stored in hot humid conditions.
U.S. Pat. No. 3,230,115 to Tamminen discloses an early example of a printed battery. Printed side-by-side on a plastic sheet are pairs of electrodes overlapped by an electrolyte layer made of a viscous adhesive gel. Conductive inks are printed in contact with outer edges of the electrodes for connecting like electrodes to each other (for forming a multi-cell battery) or to an electrical load. A plastic adhesive paint is sprayed over the electrolyte layer to prevent evaporation of moisture while venting gas byproducts.
A battery laminate disclosed in U.S. Pat. No. 5,350,645 to Lake et al. features electrolyte and electrode layers printed on or otherwise applied to sheets that are stacked together to form batteries. U.S. Pat. No. 5,652,043 to Nitzan treats the problem of liquid evaporation from the electrolyte layer of a similar stacked electrochemical cell laminate by adding a deliquescent material to the electrolyte layer.
Although various steps are taken by Tamminen, Lake et al., and Nitzan to prevent deterioration of electrolyte, their printed electrochemical cells are still subject to discharge through electrolyte layers connecting electrodes. Since the capacities of printed electrochemical cells are generally limited, such self-discharging can significantly limit their shelf life.
Regardless of cell shape, confining liquid electrolyte has long been a problem. U.S. Pat. No. 5,225,291 to Rao solves this problem in marine batteries by using surrounding sea water as an electrolyte. Electrode plates are mounted on opposite sides of a dielectric plate. Activation of the cell is deferred until the plate assembly is immersed in sea water.
Our invention is primarily directed to extending the shelf life of printed electrochemical cells but is also useful as a switching mechanism for activating the electrochemical cells. An electrolyte layer of the printed electrochemical cells is maintained out of ionic communication with at least one of two electrode layers until the electrochemical cells are placed in service. Various types of electrochemical cells can benefit from this invention including power cells and display cells.
One example of a printed electrochemical cell arranged for deferred assembly includes the usual combination of first and second electrode layers and an electrolyte layer. An electronically conductive pathway is arranged to support a flow of current between the electrode layers. However, in contrast to a fully assembled electrochemical cell, the electrolyte layer is positioned out of contact with at least one of the electrode layers for interrupting an ionically conductive pathway between the first and second electrode layers. A protective layer temporarily covers the electrolyte layer prior to completion of the cell assembly. The interruption of the ionically conductive pathway greatly reduces possibilities for self-discharge of the cell. However, when ready for service, the electrolyte layer can be separated from the protective layer and positioned in operative contact with both electrode layers to close the ionically conductive pathway.
The two electrode layers, the electrolyte layer, and the protective layer can be supported in various combinations on the same or different substrate portions (which include different substrates or different parts of the same substrate). For example, the two electrode layers can be stacked or laid out side-by-side on a first substrate portion, the electrolyte layer can be laid out on a second substrate portion, and the protective layer can be laid out on a third substrate portion. For purposes of storage prior to completion of the cell assembly, the second and third substrate portions are laminated togetherxe2x80x94the electrolyte layer in contact with the protective layer. For completing cell assembly and placing the cell in service, the second and third substrate portions are separated and the first and second substrate portions are then laminated togetherxe2x80x94the electrolyte layer in contact with the first and second electrode layers.
The two electrode layers can also be laid out on different substrate portions. For example, one of the electrodes can be laid down on a first substrate portion, and the other electrode can be laid down together with the electrolyte layer on a second substrate portion. A protective layer of a third substrate portion is initially laminated to the second substrate portion for covering an otherwise exposed surface of the electrolyte layer. Upon separation from the third substrate portion, the second substrate portion can be laminated to the first substrate portion to complete the cell.
The different portions of a single substrate can be variously laminated together by separating, folding, or winding the substrate. For example, both electrodes, the electrolyte layer, and the protective layer can be arranged on a single substrate that is separated into parts and stacked together, folded, wound, or otherwise manipulated so that the electrolyte layer is alternately positioned in contact with the protective layer or the two electrodes.
Preferably, the electrolyte of the electrolyte layer is formed by a pressure-sensitive electrolytic adhesive, and the protective layer is formed by a release liner. After removal from the release liner, the electrolytic adhesive can be used to bond the two electrodes and their supporting substrate portions together in positions that complete the ionically conductive pathway between the electrodes.
Another example of an electrochemical cell arranged for deferred assembly includes an electrolyte layer that is mounted on a different substrate portion than at least one of two electrode layers but is stacked together with the two electrode layers between the different substrate portions. For instance, a first electrode layer can be mounted on a first substrate portion, a second electrode layer together with an electrolyte layer can be mounted on a second substrate portion, and the two substrate portions can be laminated together. An electronically conductive pathway interconnects the first and second electrode layers. However, a separator mounted between the electrolyte layer and the first electrode layer interrupts an ionically conductive pathway between the first and second electrode layers.
The separator can be formed by a removable substrate, a substrate with an opening through which the ionically conductive pathway can be completed, or a passivating covering for the electrolyte. The removable substrate, which can include a release layer (especially if the electrolyte layer is formed by a pressure-sensitive adhesive), isolates the electrolyte layer from the first electrode layer but can be removed from the stack to permit the electrolyte layer to contact the first electrode layer.
The substrate with an opening functions as a spacer. The substrate portions supporting the electrolyte layer and the first electrode layer are spaced apart on opposite sides of the spacer opening. However, the substrate portions, which are also resilient, can be pressed together to move the electrolyte layer into operative contact with the first electrode layer within the spacer opening. The electrolyte is preferably a pressure-sensitive electrolytic adhesive for bonding with the first electrode layer, and the first electrode layer is preferably flexibly mounted from its underlying substrate portion so that the first electrode layer remains bonded to the electrolyte layer even after the supporting substrate portions return to their original positions.
The passivating covering preferably includes an ionically inactive coating for encapsulating (e.g., microencapsulating) the electrolyte. A mechanical action such as rubbing, scratching, or squeezing the substrate portions can transfer sufficient force to release the electrolyte from its encapsulation and complete the ionically conductive pathway between electrodes.
One more example of an electrochemical cell arranged for deferred assembly includes first and second electrode layers and an electrolyte layer that is mounted out of ionic communication with the first electrode layer. The electrolyte layer is transformable from a material that exhibits a lower adhesion to the first electrode layer to a material that exhibits a higher adhesion to the first electrode layer. Upon the transformation, the electrolyte layer is mountable in ionic communication with the first electrode layer for completing the ionically conductive pathway between the first and second electrode layers.
Initially, the electrolyte layer is preferably mounted out of contact with the first electrode layer for interrupting the ionically conductive pathway between the first and second electrode layers. However, after activation such as by wetting, the electrolyte layer is preferably movable into operative contact with the first electrode layer for completing the ionically conductive pathway between the first and second electrode layers. The contact can also provide a bond between the first and second electrode layers.
Our invention can also be arranged as a succession of deferred assembly electrochemical cells. The first and second electrode layers are printed in repeating patterns along one or more webs, and an electrolyte layer is printed out of contact with at least one of the first and second electrode layers for interrupting a succession of ionically conductive pathways between the repeating patterns of the first and second electrodes. However, the electrolyte layer is movable together with underlying portions of the one or more webs for completing the ionically conductive pathways between the repeating patterns of the first and second electrode layers.
The electrolyte layer is preferably made of an electrolytic adhesive and is protected by a release layer. Portions of the electrolyte layer are releasable from the release layer and movable into operative contact with the repeating patterns of the electrode layer for subsequently completing the ionically conductive pathways between the repeating patterns of the first and second electrode layers. Conductors extending from the succession of cells can be connected to the same or different loads (e.g., in series or in parallel).
A succession of the printed electrochemical cells can also be arranged in an array of separately actuatable power sources connected to a common load circuit. The electrochemical cells have electronically conductive pathways extending from pairs of electrodes and arranged in parallel with the common load circuit. At least one member of each of the pairs of electrodes is mounted out of contact with an electrolyte. However, the electrolyte can be moved into operative contact with the electrodes of each pair for individually activating the electrochemical cells. Thus, the array provides a succession of power sources that can be activated in sequence for maintaining or restoring power to a common load circuit.
In addition to providing a source of power, the electrochemical cells of our invention can also be arranged as display cells with deferred activation. The display cells include first and second electrode layers and an electrolyte layer positioned out of contact with at least the first electrode layer for interrupting an ionically conductive pathway between the first and second electrode layers. The electrolyte layer, which can be supported by a separate substrate portion from the first electrode layer, is movable together with the separate substrate portion into operative contact with the first electrode layer for initiating an electrochemical reaction that erodes the first electrode layer and displays visual information.
Our invention can also be practiced as method of constructing a succession of electrochemical cells arranged for deferred assembly along an in-line press. The method involves advancing first and second electrode layers along the press with at least one of the first and second electrode layers being printed in a repeating pattern. An electrolyte layer is applied out of contact with at least the first electrode layer for interrupting an ionically conductive pathway between the first and second electrode layers. A finishing step arranges the electrode and electrolyte layers into one or more formats while maintaining the electrolyte layer out of contact with at least the first electrode layer.
The electrolyte layer is preferably made of an electrolytic adhesive, and a release layer is preferably mounted over the electrolyte layer in advance of the finishing step. The finishing step can include winding the first and second electrode layers and the electrolyte layer into one or more rolls, fan folding the same layers into one or more stacks, or die cutting to separate the succession of electrochemical cells formed by the layers.